In the hydrotreating process, hydrocarbon oils are hydrogenated, desulfurized, denitrified, cracked, or otherwise treated in the presence of hydrogen. For such a process, a catalyst is used in a carrier made from an inorganic oxide, such as alumina, silica alumina, or titania. The catalyst contains at least one kind of metal selected from Groups VI and VIII of the Periodic Table to promote hydrotreating. Mo and W are frequently used as Group VI metals. Co and Ni are often employed as Group VIII metals.
Usually, these metals are contained in the form of oxides. Since they are not active as they are, it is necessary to transform the oxides into sulfides to activate them before they are used in a hydrotreating process, i.e., presulfiding is necessitated.
Generally, presulfiding is effected by passing a sulfurizing agent through a catalyst bed together with hydrogen after the catalyst is loaded into the reactor in which hydrocarbon oil is to be hydrotreated. The operating conditions of the preliminary sulfurization differ depending on the hydrotreating process and also on the sulfurizing agent used. Where hydrogen sulfide is used, about 0.5 to 5% by volume of hydrogen sulfide is contained in hydrogen, and 1000 to 3000 l (at normal temperature and normal pressure) of this compound is employed per liter of the catalyst. The temperature is in excess of 180.degree. C. Normally, the temperature is higher than 250.degree. C. Where carbon disulfide, normal butyl mercaptan, dimethyl sulfide, dimethyl disulfide, or other similar material is utilized, it is diluted with a light hydrocarbon oil. The conditions are as follows. The temperature is 250.degree. to 350.degree. C. The pressure is 20 to 100 Kg/cm.sup.2. The liquid space velocity is 0.5 to 2 hr.sup.-1. The ratio of hydrogen to oil is 200 to 1000Nl/l. After the presulfiding process is carried out in this way, the feedstock to be treated is introduced, and the hydrotreating process is initiated.
The aforementioned presulfiding process determines whether the subsequent hydrotreating process succeeds and, therefore, appropriate selection of the used materials and careful operations are needed. As an example, when a diluting agent is used, if it contains olefins, then the resulting polymers poisons the catalyst. For this reason, it is necessary to employ a hydrocarbon oil containing no olefins. Where the viscosity is high, the wettability on the surface of the catalyst is low and so heavy oils are inappropriate. Consequently, the use of light distillates is required. This results in an increase in cost. If a metal catalyst reacts with hydrogen at a high temperature and is reduced, then it is passivated. To prevent this undesirable phenomenon, a large amount of sulfurizing agent must be used. Also, the ratio of hydrogen to the sulfurizing agent must be maintained at appropriate values. Usually, the presulfiding of this kind is carried out for several days. Because this operation is temporary, it is rarely automated. Thus, unusally complex operations are required. This imposes a great burden on the operator. Hence, the omission of the presulfiding step or, at least, a decrease in the complexity of the operations has been required.